Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics. Such compositions are generally provided as imageable layers.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available imagesetters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
A contrast dye may be added to imageable elements to improve differentiation between exposed and non-exposed areas, to have a deep contrast in the exposed areas, to have at least some contrast between exposed and no-image area for baked plates, to have a certain contrast during inline manufacturing for maintaining high quality and yield, and to obtain a contrast between exposed and non-exposed areas for easier detection of malfunctions of the exposed material.
Thus, the contrast dye should exhibit a large extinction coefficient to obtain a deep contrast, be inert and not participate in material transformation induced by light absorption, and be easy to disperse in an alkaline processing solution with a pH<10 upon high loading, which facilitates these materials for simple processing.
Imageable elements containing photopolymers have become a likely use for contrast dyes. In particular, digital imaging in the violet or near-infrared region requires mostly addition of a contrast dye for some of the reasons mentioned above. Heterogeneous systems may include contrast pigments or homogeneous systems may include a soluble contrast dye. The use of the soluble contrast dyes make it more possible to process the imaged elements using an alkaline pH developer having a pH<10 and therefore it opens the possibility of simple processing. This could result in the use of a single processing solution for both violet and near-IR imaged elements.
Pigments have become very attractive as contrast materials for photopolymers despite some disadvantages. The use of pigments as contrast materials is described for example in Japanese Kokai 2002-189295 and EP 1,070,990 (Sorori et al.) and 1,520,695 (Goto) Many attempts to replace the pigment with either a water or organic solvent soluble dye have failed. The use of a water-soluble contrast dye is favored instead of a pigment because pigments tend to agglomerate in processing solutions, resulting in undesired sludge and high loading in the processor that requires frequent cleaning and shorter loading cycles.
WO 2008/046775 (Callant et al.) describes the use of water-soluble contrast dyes in lithographic printing plates that use the coalescence of particles to provide an image. The speed of such printing plates is undesirably slow. Printing plates that use coalescence to form an image only require thermal irradiation, such as near-IR lasers.